The Photometric Performance and Calibration of WFPC2

Abstract

We discuss the photometric performance and calibration of the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The stability and accuracy of WFPC2 photometric measurements is discussed, with particular attention given to charge-transfer efficiency (CTE) effects, contamination effects in the ultraviolet (UV), and flat-field accuracy and normalization. Observational data are presented from both WFPC2 observations and ground observations using a system similar to that flown. WFPC2 photometric systems are defined both for the ground and flight systems. Transformations between these systems and the Landolt UBVRI system are presented. These transformations are sensitive to details in the spectra being transformed, and these sensitivities are quantified and discussed. On-orbit observations are used to revise the prelaunch estimates of response curves to best match synthetic photometry results with observations, and the accuracy of the resulting synthetic photometry is discussed. Synthetic photometry is used to determine zero points and transformations for all of the flight filters, and also to derive interstellar extinction values for the WFPC2 system. Using stellar interior and atmosphere models, isochrones in the WFPC2 system are calculated and compared with several observations.

The Metallicity Dependence of the Stellar Luminosity and Initial Mass Function: HST Observations of Open and Globular Clusters

“…The temperature change was made in order to decrease a charge-transfer efficiency (CTE) problem which manifested itself as a position-dependent non-linearity of the WFPC2 CCDs. The effect was found to be ≈ 10% at −76 • C and ≈ 4% at −88 • C (Holtzman et al 1995b) in a series of 40 second calibration exposures taken of a field in ω Cen. Elevated sky counts found in longer exposures such as ours are expected to decrease the CTE problem (Holtzman et al 1995a).…”

“…5) The precision of the Holtzman et al (1995b) transformation is expected to be ≈ 2%, over the applicable color range. The range of applicability of the Holtzman et al photometric transformations is −0.3 < V-I < 1.5, whereas our stars continue to V-I ≈ 3.5.…”

The Age, Mass, and Size Distributions of Star Clusters in M51

“…This filter contains both stellar continuum (observed for clusters at all ages) and nebular line (observed only for clusters with ages  t 10 Myr) emission, which allows us to use this measurement in the spectral energy distribution fits to determine cluster ages and masses of each cluster (Section3). The instrumental magnitudes were converted to the VEGA-MAG photometric system by applying the zeropoints given in Sirianni et al (2005) for the ACS filters, and those given in Holtzmann et al (1995) for the WFPC2 filter. We measure the concentration index (CI, defined as the difference in the magnitude measured within a 3 and 0.5 pixel radius) for each cluster, as an estimate of its size.…”

The Size and Shape of the Oblong Dwarf Planet Haumea

“…In five of our images the primary is contaminated by cosmic rays so we do not include these data. A 0.5" aperture is used to measure the object, and we apply an infinite aperture correction of 0.1 magnitudes (Holtzman et al 1995). We present and model the data in the STMAG magnitude system, but a convolution of the F606W filter with the We combine our data with that of Rabinowitz et al (2006) and Lacerda et al (2008) to get a 4-year baseline of observations and find a period of 3.91531 ± 0.00005 hours using phase dispersion minimization (Stellingwerf 1978).…”